1. Field of the Invention
The present invention relates to active matrix liquid crystal display devices incorporating therein driver circuits, and more particularly to a technology to enhance definition and image quality for the liquid crystal display devices.
2. Description of Related Art
In recent years, technological developments have being put forwarded in flat panels, such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescence (EL) displays, as cathode ray tubes (CRT) replacing displays. Among these flat displays, liquid crystal displays are the largest in marketplace and utilized for various display mediums including notebook personal computers, digital cameras with liquid crystal panels, car navigation systems, projectors and wide screen televisions.
The advantage of the liquid crystal display greater than the CRT lies in that the display area is obtained wide due to display section flatness and high definition given by the dot matrix display scheme.
The high definition is meant to increase in the number of pixels in the liquid crystal display. A drive frequency increases with increase in the number of pixels. For example, the number of pixels, although about four hundreds of thousands in NTSC rating, mounts to approximately two millions (1920xc3x971080 pixels), in HDTV rating. Accordingly, in HDTV rating the input video signal has its maximum frequency reaching as high as 20 to 30 MHz, despite it was 6 MHz in the NTSC rating.
In order to display video signals with accuracy, a clock signal requires a frequency of several times (e.g., about 50 to 60 MHz) that of the video signal. It is expected that display with higher definition and image quality be furthermore required from now on and video signals with a dot clock extremely high in speed be dealt with.
FIG. 11A shows a simplified routes for video signals to be inputted to the conventional liquid crystal display panel. The liquid crystal display panel 10 is arranged, as shown in FIG. 11A, with a pixel matrix area 11, and a gate driver circuit 12 and a source driver circuit 13. The gate driver circuit 12 is also called a scanning line driver circuit. The source driver circuit 13 is also called a signal line driver circuit or a data line driver circuit. The pixel matrix area 11 has pixels, each pixels having a liquid crystal cell 15 and a pixel TFT 16. The liquid crystal cell 15 possesses a capacitor structure having dielectric sandwiched between a pixel electrode to be inputted by a video signal and an opposite electrode. The pixel TFT 16 includes a gate electrode, a source electrode and a drain electrode. The gate electrode is connected to a scanning line 17, the source electrode (or the drain electrode) is connected to a signal line 18 and the drain electrode (or the source electrode) is connected to the pixel electrode of the liquid crystal cell 15. The scanning line 17 is connected to the gate driver circuit 12 and the signal line 18 is connected to the source driver circuit 13. The scanning line 17 is also called a gate line. The signal line 18 is also called a data line, a source line or a drain line.
The video signal to be applied to the pixel cell is processed suitably for display characteristics of the liquid crystal panel 10 by the video signal processing circuit 20. The video signal processing circuit 20 mainly performs gamma correction, alternation and amplification to process on video signals inputted from the outside. The processed video signal is inputted from the source driver circuit 13 through the signal line 18 to the pixel matrix area 11, thus applied to the pixel electrode of the liquid crystal cell 15. The liquid crystal material in the liquid crystal cell 15 varies in light transmission rate depending upon a voltage applied to. The change of light transmission rate corresponds to tone whereby images are formed by the entire liquid crystal cells 15.
In order to realize high quality display on the liquid crystal panel, the video signal processing circuit 20 requires an amplifier 21 (see FIG. 11B) to amplify signal waveforms with fidelity. This is because the amplifier 21 is at a final output end of the video signal processing circuit 20 where the video signal to be applied to the pixel electrode of the liquid crystal cell 15 is finally determined in amplitude and form. The video signal applied to the pixel electrode is a pulse-formed signal. Consequently, the amplifier 21 is required not to cause pulse signal amplitude deterioration and rounding of pulse waveforms.
It is known that the amplifier 21 generally has a frequency characteristic as shown in numeral 1101 of FIG. 11C wherein a voltage gain is nearly constant in a middle range but, in a range exceeding a certain frequency, decreases at a constant rate. The decrease rate is xe2x88x9220 dB/decade (xe2x88x926 dB/octave) where the amplifier is in one stage. The cause of decreasing the gain in the higher range is due to output impedance increase in the single amplifier.
In the liquid crystal display, however, consideration has to be given not only to the output end voltage of the amplifier 21 but also to the voltage finally applied to the pixel electrode. Accordingly, there is a necessity for the frequency characteristic of the amplifier 21 in the video signal processing circuit to consider also the resistance RLC and capacitance CLC connected between the amplifier 21 and the liquid crystal cell 15 instead of the single amplifier 21. Thereupon, as shown in numeral 1102 of the FIG. 11C the frequency range in which the gain of the pixel electrode of the liquid crystal cell 15 begins to lower is shifted to a lower side than the gain of the single amplifier 21 by impedance decrease due to the liquid crystal panel resistance RLC and capacitance CLC.
The increase of definition in the liquid crystal display is pixel and pixel density increase. The pixels, if increased, increases the number of connection lines, increasing liquid panel resistance RLC. The density increase actualizes; the problem of pixel matrix parasitic capacitance, giving rise to a tendency of increasing the capacitance CLC. Accordingly, the increase of definition results in a shift of the frequency range in which the gain of the amplifier 21 is flat toward the lower range side. In order to avoid the gain decrease, the resistance RLC may be decreased. In order to reduce the resistance RLC, the thickness of interconnection may be increased. However, the increase in interconnection thickness leads to increase in interconnect occupation area, running counter to a direction of a technological development called pixel shrinkage.
The increase in definition also requires high frequency drive. The video signal drive frequency in the HDTV rating requires as high as 20 to 30 MHz. If an HDTV rating display is realized by a liquid crystal panel, the video signal frequency fvid unavoidably comes to a frequency range that the gain on the pixel electrode is decreased due to the above-described increase in definition of the liquid crystal panel.
If a gain decrease on the pixel electrode occurs in the video signal frequency fvid, the video signal decreases in black or white level, resulting in image graying (muddy color in color display) and hence degradation in display quality.
High frequency drive has been unnecessary for such a VGA or SVGA rated liquid crystal panel as having the horizontal number of pixels of less than a thousand. Consequently, even if there has been a decrease on the high frequency side in the gain of the voltage applied to the pixel electrode, the amplifier 21 could be used at a frequency at which the gain is flat. The problem of the gain decrease concerning the frequency has not been recognized at all.
It is an object of the present invention to provide a liquid crystal display device which is capable of displaying with high quality, wherein the gain reduction in the high frequency range is compensated for the video signal to be applied to pixel electrodes of a pixel matrix area to eliminate the above-described problem due to increase in definition for the display device.
According to a structure of the present invention, a liquid crystal display device, at least comprises: a pixel matrix area having a switching element for each pixel electrode; a first driver circuit connected to scanning lines of the pixel matrix area; a second driver circuit connected to signal lines of the pixel matrix area; a video signal processing circuit for alternating video signals and outputting a plurality of alternating video signals onto the second driver circuit; and a control circuit for creating control signals to control on drive to the first driver circuit, the second driver circuit and the video signal processing circuit; wherein the video signal processing circuit has a circuit for effecting a peaking process connected to an output of an amplifier placed at the closest to each output terminal outputting the alternating current video signals.
According to another structure of the invention, the video signal processing circuit converts the video signals into the alternating current video signal and outputs the alternating current video signals to the second driver circuit. The alternating current signals are constituted by two kinds of alternating current signals in an inverted relation to each other. The video signal processing circuit has a circuit for effecting a peaking process connected to an output of an amplifier placed at the closest to each output terminal outputting the alternating current video signals.
In the liquid crystal display device of the invention, a peaking processing circuit is connected to an output of the amplifier placed at the closest to an output terminal outputting the video signals. This makes it possible to display in high definition display by compensating for voltage gain on the pixel electrodes due to reduction in impedance loaded on the amplifier, i.e., impedance of the pixel matrix area or driver circuit.